The invention disclosed herein is an audio amplification system which features protection against damage to the hearing of persons who might otherwise be exposed to sound levels in excess of acceptable levels.
It is common practice at indoor or outdoor music concerts for the performers to use a sound amplification system which drives speakers at sound output levels which can damage the hearing of the listeners. The health authorities of governments have set up noise level limitation standards whose purpose it is to preserve the hearing integrity of employees. The Occupational Safety and Health Administration (OSHA) of the United States Government, the American National Standard Institute (ANSI) and the International Standards Organization (ISO) have established standards for protection of employees against the effects of noise exposure. The OSHA standards are published in 29 C.F.R. 1910.95. Basically, the standards provide for an inverse relationship between the number of hours of exposure to the noise per day and the level of the noise or sound in terms of decibels. For example, the safe limit for an employee exposed to sound levels at 95 decibels is 4 hours whereas if the sound level is at 110 decibels the limit of exposure is 1/2 hour. The standards also recognize that it is not only the total sound energy or level which must be taken into consideration but also the frequency content of the sound or noise. Generally, sounds in the higher frequency part of the audible frequency range are more energetic and can cause greater damage at a given decibel level than sounds at much lower frequencies. For example, a very high decibel level is permissible for sounds near the lower frequency part of the audible range such as around 20 Hz whereas a much lower decibel level is permissible for sound having the same energy content at a higher frequency such as 4000 Hz or above. It is known that sound levels in the higher frequency parts of the spectrum which often occur at so called rock music concerts cause permanent damage to the hearing of listeners. It is also well known that the cumulative sound dose at many concerts exceeds the permissible limits for total dosage set by the various standards.
Besides taking measures to protect the hearing of members of the audience who have paid an admission fee for listening to a concert there must be concern about the possible damage to the hearing of employees such as the musicians, audio amplifier operators and those who are compelled to attend the concert such as officials who are there to prevent unlawful and disorderly conduct. Audio dosimeters attached to employees who are exposed to whatever sound is emitted by the speakers have demonstrated huge overdoses of sound during a concert. It has been discovered that the persons who regularly operate the amplification equipment at concerts have suffered severe hearing loss without appreciating it. Since their hearing is subnormal they have a tendency to turn up the gain of the amplifiers to a level which they think is tolerable because of their hearing deficiency but which will result in the onset of permanent hearing damage to the listeners.
There are known systems for achieving automatic volume control for the output volume from a set of loudspeakers. Examples are given in U.S. Pat. Nos. 2,338,551; 2,468,205; 2,616,971; 3,009,991; 4,254,303; and 4,306,115. These patents adjust the sound output level from the speakers to compensate for background noise level. In other words, if there is a high background noise, the gain of the amplifier system is turned up so that the useful sound can be heard over the noise level. As the noise level subsides, there is an automatic reduction of the sound output from the speakers. Another U.S. Pat. No. 4,583,245 discloses a system for protecting the audio output speakers from damage that might be caused by driving a speaker with sound containing frequencies which cause the speaker diaphragm to execute excursions which are greater than the diaphragm was designed to withstand. Thus, in this patent, the input sound spectrum from the microphones is separated into its constituent frequency bands. In this way the frequency for which a particular speaker has been optimized is directed to that speaker so the speaker will not be damaged. The listener's ears reconstitute the sound spectrum which is emitted by the separate speakers.
Up to the time that the invention disclosed herein was made, no one has provided an audio control system which automatically brings about compliance with established standards which set the limits for impulse or instantaneous sound levels and cumulative dosage as well.